Tech Forum

You don’t need a converter. You need an inverter. A converter is used to change AC into DC or DC of another voltage, while an inverter changes DC to AC. That being said, there are many ways to skin this cat, depending on how much you want to spend and what you want to use the AC output for. Before we go there though, we have to start with basics: the generator.

In your question, you did not specify the type of DC generator that you have or how you are driving it. There are three types: the shunt; the series; and the compound. Each has its own set of operating characteristics. I assumed that yours is of the shunt type, the most common. The Rheostat (R) is used to adjust the output. The voltage (E) and current (I) are monitored and the output is supplied through a 125 amp fuse.

Your generator is rated for 100 amps at 110 volts which is 11,000 watts (11,000/750 = 14.67 horsepower). Add 20% because no generator is 100% efficient, so about 18 horsepower is required to drive it at its rated speed. This is important because the output of a DC generator is related to both its field strength and operating speed.

There are three ways to do what you want to do.

The first option is the simplest — just use a 110 volt DC to AC inverter. These are around because they were once used on small hydro electric systems. You may be able to find one on the Internet, but I don’t recall any being large enough to handle 11,000 watts.

The second option is a bit more radical, but much more feasible as the inverters are widely available for usage in the alternative energy field. A pair of 48 volt magnum energy MS4448PAE inverters are connected in series and these are in parallel with a pair of series connected high-value capacitors. An equalizer line runs from the junction of the capacitors to the junction of the inverters. This works because these inverters like to operate around 55 volts and because their outputs are in parallel, they share the load evenly. The equalizer line is a safety to handle any imbalance.

As for output, these inverters are rated for 4,400 watts continuously, 4,800 watts for 30 minutes, and 6,000 watts for 30 seconds. The first two specifications are all that matter in your application. Assuming 90% inverter efficiency, the generator will supply 9,680 watts (88 amps) to produce 8,800 watts of AC. The generator will supply 10,560 watts (96 amps) to produce 9,600 watts of AC for 30 minutes at a time.

The third option is the most radical, but it allows two things the other options do not: redundancy and the ability to supply AC to the utility grid. This option utilizes microcontrollers commonly used on photovoltaic modules to convert low voltage DC into 60 Hz and then to synchronize this to the utility grid. I have chosen the Enphase Energy 250 watt unit. You will need 36 of them and they are used in groups of three to allow an input of 48 to 144 volts. This is well within the range of your generator.

The outputs of all the units can be connected together to produce about 9,000 watts of AC power at 120/240 volts or they may be connected for three-phase output at 208/120 volts. They must be connected to the AC grid to operate though. By having 36 units in 12 series strings, the failure of a single unit will only reduce the output by a little more than 8%. It was nice talking about the good ole days of DC. If you have any other questions, you can contact me directly at PO Box 9106, Concord, MA 01741.